Which of these numbers is prime? ${24,\ 25,\ 77,\ 85,\ 97}$
Explanation: A prime number has exactly two factors: 1 and itself. The factors of 24 are 1, 2, 3, 4, 6, 8, 12, and 24. The factors of 25 are 1, 5, and 25. The factors of 77 are 1, 7, 11, and 77. The factors of 85 are 1, 5, 17, and 85. The factors of 97 are 1 and 97. Thus, 97 is a prime number.